1. Technical Field
The present invention relates to a hole transport polymeric compound, which has excellent hole transport capabilities, exhibits stability in organic solvents so as to be insoluble in the organic solvents used upon stacking other organic layers, and can efficiently block electrons, and a polymer light emitting diode using the same.
2. Description of the Related Art
In modern fields such as IT and electronics, photo-electronic devices based on conversion of light energy into electric energy or vice versa are very important.
With various merits, such as easy and inexpensive solution process, excellent luminance, rapid response, flexibility, and the like, polymer light emitting diodes (PLEDs) have been actively studied for development of next generation flat panel displays.
Efficiency of PLEDs substantially depends on charge equilibrium, electron blocking and exciton quenching properties. These requirements are achieved by a multilayer structure including functional layers, such as a hole injecting layer (HIL), hole transport layer (HTL), electron blocking layer (EBL), electron emission layer (EML), hole blocking layer (HBL), and electron transfer layer (ETL). Particularly, to realize highly efficient PLEDs, there is a need for the development of an appropriate hole transport layer, for confining excitons in the emission layer by maximizing hole injection and electron blocking. In addition, the hole injection layer serves to prevent exciton quenching near an interface between the emission layer and an electrode, thereby improving PLED efficiency.
Manufacture of PLEDs having such a multilayer structure is difficult as compared with organic light emitting diodes because previously formed layers can be damaged or dissolved by a solvent used for spin coating of a new layer.
Therefore, materials having excellent conductivity and providing suitable solubility, such as poly(3,4-ethylenedioxythiophen):poly(styrene sulfonate) (PEDOT:PSS), have been widely used for the hole injection layer and the hole transport layer of the PLED. PEDOT:PSS has many disadvantages such as deficiency in interfacial adhesion, hole transport capabilities and electron blocking capabilities, exciton quenching at the interface of the emission layer, and the like. Moreover, PEDOT:PSS has acidity and corrodes a surface of indium tin oxide (ITO), thereby causing serious problems in terms of long-term stability of the PLED.
To overcome these problems, Morgado et al. reported that insertion of poly(p-phenylenevinylene) (PPV) between PEDOT:PSS and an emission layer resulted in increase in efficiency of a fluorine-based PLED by almost two times (J. Morgado, R. H. Friend, F. Cacialli, Appl. Phys. Lett. 2002, 80, 2436.), and Kim et al. reported that formation of a thin (˜10 nm) semiconducting polymer interlayer (poly(2,7-(9,9-di-n-octylfluorene)-alt-(1,4-phenylene-((4-sec-butylphenyl)imino)-1,4-phenylene) (TFB)) between the PEDOT:PSS and the emission layer resulted in substantial increase in efficiency of the PLED (J.-S. Kim, R. H. Friend, I. Grizzi, J. H. Burroughes, Appl. Phys. Lett. 2005, 87, 023506.).
However, although efficiency and lifespan of PLEDs could be improved by overcoming the disadvantages of PEDOT:PSS, there are problems in that insertion of an intermediate layer entails complex processes and increases manufacturing costs.
Therefore, there is a need for novel hole transport materials, which have excellent hole transport capability, and exhibit stability in organic solvents so as to be insoluble in organic solvents used upon stacking other organic layers.